Method of separating a noncondensable gas from a condensable vapor

ABSTRACT

A noncondensable gas such as H 2  S is removed from a condensable vapor in a two-sided heat exchanger employing multi-stage condensation or counter current flow, and the condensate so formed is employed as a cooling medium on the opposite side of the heat exchanger while itself being vaporized into a clean gas. In multi-stage condensation, a portion of the condensate is formed at each stage of a system and the condensate so formed does not progress to the next stage. In a counter current flow system, the condensate is at least temporarily retained in the condensing area so that additional noncondensable gas may be stripped from the condensate by the gaseous mixture of noncondensable gas and condensable vapor. Before condensate from either type of system is employed as a coolant, it is subjected to flash evaporation.

BACKGROUND OF THE INVENTION

This application is a continuation of application Ser. No. 311,810,filed Oct. 15, 1981, now abandoned, which was a continuation ofapplication Ser. No. 138,007, filed Apr. 7, 1980, now U.S. Pat. No.4,330,007.

1. Field of the Invention

The invention relates to gas separation with heating or cooling meansfor gas, with two confined fluids in indirect contact; and also toprocesses for gas separation of vapors and gases, especially vapors ofsulfur and its compounds. The invention also relates to a system toseparate noncondensable gases from condensable gases or vapors and toequipment to accomplish this separation. In particular, the invention isfor removal of hydrogen sulfide gas from geothermal steam.

2. Description of the Prior Art

Geothermal steam is produced in many parts of the world and harnessedfor generation of electricity, among other useful ends. The pressurizedsteam contains a variety of other noncondensable gases that may includecarbon dioxide, ammonia, nitrogen, hydrogen, hydrocarbons, and hydrogensulfide. After the steam passes through a turbine for generation ofelectricity it may be either condensed or discharged directly to theatmosphere. The various gases are partially or completely liberated tothe atmosphere either directly from the condenser or during laterprocessing of the condensate from the condenser. The H₂ S gas soliberated is a pollutant and is undesirable in high localconcentrations.

Hot geothermal brines and water are also produced for generation ofelectricity and other purposes. These brines and waters may also containdissolved gases including H₂ S gas. The utilization of these hot watersoften involves a reduction of pressure so that part of the water flashesto steam. When this occurs, much and probably almost all of the H₂ S,depending on the specific process conditions, will transfer into thesteam phase. The potential pollution problems when the steam isprocessed then become similar to those when dry steam, as opposed to hotliquid, is produced from a geothermal well.

Air pollution problems due to the release of H₂ S gas from chemicalprocesses have been long present and recognized. This is particularlytrue in the petroleum and petrochemical industries. A large number ofprocesses for H₂ S removal have been developed and put into use invarious commercial industries. However, none of these have been found tobe particularly applicable to geothermal steam.

Several recent research efforts have attempted to remove H₂ S fromgeothermal steam or liquid. These include absorption into copper sulfateand similar solutions, and direct oxidation by adding oxygen to aliquid, H₂ S-bearing stream. No commercially acceptable process is knownto have resulted from these efforts.

With respect to the abatement of H₂ S emissions resulting from the useof geothermal steam, hot brines, or hot water, two specific applicationsof the present invention are envisioned, as described below.

When a geothermal well is drilled, whether it is in a new area or in anextension of an established well field, it is often necessary to testthe well for an extended period of time. This testing generally consistsof blowing steam to the atmosphere with the well bore completely open,or partially "choked" to restrict flow, and has such purposes as toestimate the size of the reservoir and the potential life of the well,as well as clean the well of loose solid matter. When testing orcleaning is satisfactorily completed, the well may be shut in until theflow from the well can be directed to a process plant. The process plantwill usually not be built until several wells have been completed andtested, so that the geothermal reservoir has been proven capable ofsustaining the process plant for a sufficiently long period of time tojustify the expense of its construction.

During the testing of the wells, the H₂ S contained in the steam isemitted directly to the atmosphere. In this mode of operation, no simpleand economic means is known to recover the H₂ S so as to prevent itsemission to the atmosphere. One aspect of this invention is to providesuch a means.

After construction of a process plant, geothermal steam from one orseveral wells will be directed to the plant. It is generally preferableto remove the H₂ S from the steam upstream of the process plant, wherethe steam is still pressurized and occupies a considerably smallervolume than it would downstream of the plant, as at the exit of aturbine. Another aspect of this invention is to provide a system forremoval of H₂ S from geothermal steam within a process plant, precedingall or most of the plant process equipment.

The processes and devices described can also be used for the removal ofvarious other noncondensable gases from other condensable gases andvapors differing considerably from geothermal steam. The process mayalso be applied downstream of other process equipment if this designconfiguration is preferred.

SUMMARY OF THE INVENTION

Noncondensable gas such as H₂ S is removed from a condensable vapor suchas steam and is concentrated into a relatively small vent gas stream,with respect to the size of the incoming steam stream, for recovery.Equipment and method are applied to removal of the gas. The steam isrouted into a heat exchanger where part of the steam is condensed on oneside of the exchanger to form a condensate containing little of the gas,and part of the steam remains a vapor containing most of the gas in moreconcentrated form. The vapor is conventionally processed or disposed ofin an acceptable way. The condensate is flashed into a cleaner liquidand associated vapor, and the cleaner liquid is directed to the oppositeside of the heat exchanger to serve as the cooling liquid for condensingincoming steam and to itself be vaporized into a clean steam to be sentto a process plant. The associated vapor may be added to the clean steamto be sent to a process plant. The flashing of the condensate may takeplace in a flash tank or in the heat exchanger itself by appropriatecontrol of pressures. In the latter instance, the associated vaporautomatically becomes mixed with the clean steam and is routed to theprocess plant.

In particular, the gas is removed by heat exchangers employingmulti-stage condensation or counter current flow. The former partiallycondenses steam at each stage of a system and the condensate so formeddoes not progress to the next stage. The heat exchanger receives thesteam through a long flow path and condensate forms on condensingelements such as cooling tubes throughout the path, with the condensatefrom each element draining from the condensing area. As before, thecondensate is re-evaporated at reduced pressure and temperature on theopposite side of the heat exchanger where it serves as the coolingliquid to continuously condense further incoming steam.

Counter current flow takes place in a heat exchanger that retains thecondensate in the condensing area for at least a limited time so thatadditional gas may be stripped from the condensate. In one embodiment,the steam is condensed inside the tubes of a non-horizontal tube bundleas the steam enters from the bottom of the tubes and exits at the top.Condensate forms along the length of each tube and flows downwardly pastthe upwardly travelling steam, thereby encountering a constantlychanging liquid-gas equilibrium favoring removal of gas from thecondensate at increasingly lower levels in each tube. The collectedcondensate is transferred by suitable means to the opposite side of theheat exchanger and is sprayed against the outside of the tubes as acoolant. In another embodiment, either vertical or horizontal heatexchanger tubes condense the steam on the outside of the tubes as steamenters the shell side of the heat exchanger from the bottom and vent gasexits the exchanger from the top. Condensate either drains down verticaltubes or drips down a series of horizontal tubes to achieve thestripping action previously mentioned.

The primary object of the invention is to create an apparatus and methodto recover noncondensable gases such as H₂ S from condensable vaporssuch as steam, concentrating the gases in a stream that can be handledby conventional means. It is an important object to create acommercially acceptable method of recovering such noncondensable gases,and a method that is adaptable to both single geothermal wells andlarger supplies of geothermal steam as might be routed to a processplant.

Another object is to create an apparatus and process that can be appliedupstream of a process plant, within a process plant, or downstream of aprocess plant, where a turbine-generator set may be considered a processplant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a typical apparatus for removal ofnoncondensable gases from condensable vapors.

FIG. 2 is a vertical cross-section of an apparatus for removingnoncondensable gases in a multi-stage process.

FIG. 3 is a cross-sectional view of the apparatus of FIG. 2, taken alongthe plane 3--3 of FIG. 2.

FIG. 4 is a vertical cross-section of an apparatus for removingnoncondensable gases in a counter current flow process on the tube sideof a heat exchanger.

FIG. 5 is a vertical cross-section of an apparatus for removingnoncondensable gases by crossflow on the shell side of a heat exchanger.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Apparatus for removal of H₂ S or other noncondensable vapors or gasesfrom geothermal steam or from other condensable vapors is illustrated inFIG. 1, wherein a heat exchanger 10 is of the type having a two sidedconfiguration, which may be referred to as a shell side and a tube side.The exchanger comprises a shell or outer wall 12 that contains a tubebundle 14 opening at each end into chambers 16 and 17. The shell isconnected to a geothermal steam inlet pipe 18, a condensate removal pipe20, and a vent gas removal pipe 22. Leading out of chamber 17 is a cleansteam exit pipe 24 for removal of gases from the tube side of theexchanger, and leading into chamber 17 is water inlet pipe 26.

Condensate removal pipe 20 leads from the shell side of the exchanger toa flash tank 28, which in turn has a flash tank vapor removal pipe 30leading to the clean steam exit pipe 24. Alternatively, by means of pipe30', the flash tank vapor can either be introduced into vent gas removalpipe 22 after a reduction in the vent gas pressure, or the flash tankvapor can be discarded. The flash tank is also connected to the flashtank condensate removal pipe 32 leading to pump 34. Water inlet pipe 26carries condensate from the pump to the tube side of the heat exchangerat chamber 17. Clean water inlet pipe 36 supplies liquid as required topipe 26.

The operation of the apparatus starts with the receipt of the steam Acontaining gases through pipe 18 at a temperature T₁. The steam entersthe heat exchanger, for example on the shell side thereof, and most ofthe steam is condensed and exits the heat exchanger as condensate streamB in pipe 20 at temperature T₂. That portion of the steam that was notcondensed exits the heat exchanger through pipe 22 as vent steam Hhaving a temperature T₃. Ordinarily T₁ is greater than T₂, which isgreater than T₃. Most of the non-condensable gases to be separated fromthe steam phase will exit in stream H, while a small portion of thesegases will be dissolved in the condensate stream B.

The majority of the noncondensable gases such as H₂ S are concentratedin the vent stream H, which may be processed by conventional means toprevent ultimate escape of H₂ S to the atmosphere. For example, stream Hmay be subjected to a Klaus process, wherein the H₂ S is converted tosulfur, which can be stored or used as feedstock for other purposes.Alternatively, the vent stream may be reinjected into the geothermalreservoir in such a manner that its recirculation into producinggeothermal wells is prevented or inhibited. Regardless of what ultimatedisposition is made of the vent stream H, the size of the H₂ S bearingstream is greatly reduced as compared to the original size of the streamA, and the resultant stream can be easily handled. Because the H₂ S hasbeen diverted from the process plant, the plant can be operated in asimplified manner with respect to noxious gas emissions abatement; also,maintenance otherwise required to combat chemical attack, or formationof scale deposits by such species as H₂ S, CO₂, etc., may besignificantly reduced in scope and frequency.

The heat exchanger 10 requires a cooling means to remove heat from thegeothermal steam in stream A so as to effect its condensation, and inaddition, a source of clean steam is required for use in the processplant in place of the original geothermal stream A. Both requirementsare met by supplying a stream of clean liquid water to the opposite sideof the heat exchanger from the geothermal steam, for example to the tubeside of the heat exchanger. The clean stream of water supplies thenecessary cooling to condense the geothermal steam while itself beingvaporized to clean steam by means of heat received from the geothermalsteam. A preferred source of the clean water is the condensate formedfrom the geothermal steam, but any exterior source of clean water can beutilized. Although the condensate will contain some dissolvedundesirable gases from stream A, the concentration of these gases issufficiently small that a pollution problem is not caused when the gasesare eventually released to the atmosphere without further treatment.

For employment of the condensate stream B as the cooling means andsource of clean steam, the condensate may be handled in either of twoways. According to the first, the stream B is directed through pipe 20to flash tank 28, which is maintained at a lower pressure than thepressure at which the geothermal steam condenses. At lower pressure, asmall part of the condensate vaporizes to form a vapor stream F attemperature T₄, while the remaining part of the condensate passesthrough pipe 32 as stream C at temperature T₄ to pump 34 after which itis fed through pipe 26 to the side of the heat exchanger 10 oppositefrom the geothermal steam stream for evaporation. The flashed steam fromtank 28 may be routed as stream F through pipe 30 to mix with the cleansteam leaving the heat exchanger through pipe 24 as stream E attemperature T₅, to form clean steam stream G at temperature T₈, which isthen routed to the process plant. The flashed steam of stream F containsmuch of the gases dissolved in the condensate stream B. Alternatively,to create a cleaner steam to be fed to the process plant the stream Fmay be discarded.

The second route for handling the condensate feeds the stream B directlyto the tube side of the heat exchanger without the necessity of theflash tank 28. The heat exchanger on the tube side is also maintained ata lower pressure than that of the shell side, with the result thatflashing of the condensate occurs in the heat exchanger itself or in thepiping upstream of it. The vapor formed in the second method passes withthe clean liquid through the heat exchanger to exit with the steamformed by evaporation of the condensate. The pump 34 may not be requiredin this instance, depending on the specific design of the heatexchanger.

In order to balance the heat load on the heat exchanger, a small makeupstream I of clean liquid water at temperature T₆ may be provided throughpipe 36 to mix with the condensate stream D being fed to the heatexchanger. The combined stream J at temperature T₇ then enters the heatexchanger at chamber 17.

The process and apparatus described in FIG. 1 will provide a cleanersteam than the geothermal steam A itself. If desired, the process mayalternatively be applied to steam leaving the process plant, such as tothe lower pressure steam leaving a turbine. In some instances, the cleansteam so produced will not be clean enough to meet desired air pollutionstandards, as for example when the geothermal steam contains largeamounts of ammonia gas in addition to the H₂ S. The basic ammoniadissolved in condensate stream B will cause a relatively greater amountof the acidic H₂ S to be absorbed into stream B than in the case whenammonia is not present. To achieve higher rates of H₂ S removal throughvent stream H and correspondingly lesser amounts absorbed in condensatestream B, special heat exchanger designs and methods of operation areemployed, as illustrated in FIGS. 2-5.

The heat exchangers of FIGS. 2-5 provide either a multi-stage systemwherein partial condensation of the steam occurs at each stage and thecondensate therein produced does not progress to the next stage, acounter current system wherein flow of condensate and steam are inopposite directions, or some combination of multi-stage and countercurrent systems.

FIG. 2 illustrates a multi-stage system wherein steam condenses in theshell side of a vertically oriented shell and tube heat exchanger. Thesteam flow path in such a heat exchanger may be either crossflow, backand forth flow, radial flow from the outer circumference to the innercircumference, radial flow from the interior of the tube bundle to theouter circumference, or a combination of these methods. For example, inFIGS. 2 and 3 the heat exchanger 40 receives the geothermal steam instream A through inlet pipe 42 leading to the shell side of theexchanger. The steam in this example flows radially inwardly, with thevent stream H exiting the exchanger at the center of the tube bundlethrough pipe 44. In FIG. 3, the steam deflector plate 43 provides evendistribution of incoming geothermal steam for uniform radial flow towardpipe 44. Condensate forms on the outside surfaces of the tubes 46 anddrains to the bottom of the shell space 48 from where it is withdrawn.The condensate formed on the first tubes to be encountered, in thisinstance the outer ring of tubes, is in contact with a relatively largevolume of steam. Thus, according to the known principals of gas-liquidequilibrium, only a small portion of the noncondensable gases will beabsorbed by the liquid of this initial condensate. At the inner ring oftubes, the mass ratio of condensate to uncondensed steam is relativelylarger, with the result that the condensate formed on the inner ring oftubes will contain a proportionately larger amount of absorbed gasesthan the condensate formed at the outer ring. The mixture of condensatefrom all the tubes will contain a smaller amount of absorbed gases thanwould be the case if all the condensate were in contact with, and inequilibrium with, the vent gas stream H. Normal non-equilibriumconditions may cause an even greater amount of absorbed gases to beremoved from the condensate.

The condensate from shell space 48 is drained through conduit 50 tolower chamber 52 on the tube side of the heat exchanger where the liquidcollects in a sump and is subsequently routed through pipe 54 to pump 56and is pumped through pipe 58 to upper chamber 60. A make up stream ofclean liquid water may be delivered to pipe 54 through pipe 62. Theliquid entering upper chamber 60 then flows down the inside wallsurfaces of the vertical tubes 46, where the liquid serves both as thecooling means for condensing part of the geothermal steam on theopposite side of the tubes, and as the source for creation of cleansteam to be routed to the process plant.

The boiling liquid inside the tubes may flow in one of several normalflow patterns, including falling film flow, upwards boiling flow withvaporization, or forced convection flow with suppressed vaporization. Inthe particular example of FIGS. 2 and 3, falling film flow isillustrated. The clean steam formed in the tubes enters the lowerchamber 52 and is carried to the process plant through pipe 64, which islocated above the condensate level in the lower chamber. As in theprevious embodiment, the pressure of the condensate is reduced beforethe condensate is used as coolant, for example by the maintenance of alower pressure in the tube side of the heat exchanger than in the shellside. A suitable valve 66 in conduit 50 can create the desired pressuredifferential. This example constitutues direct transfer of condensatefrom the shell side to the tube side of the heat exchanger.

Counter current flow systems may condense geothermal steam either insideor outside the tubes, and the heat exchanger may be oriented in anyposition between vertical and horizontal. It is necessary in a countercurrent flow system that the inlet geothermal steam flows in theopposite direction from the direction of condensate movement, with theinlet steam in direct contact with the condensate. Examples of countercurrent condensation include the heat exchanger 70 of FIG. 4, whereinthe geothermal steam stream A enters the lower chamber 72 (through pipe71) above the liquid level in sump 74 and passes into the upward slopingtubes 76 in the tube side of the exchanger. As the steam passes up thetubes, the condensable portion condenses on the tube walls and bygravity drains down the tube walls to the sump 74 counter to the flow ofsteam. The condensate is cleaned during its downflow by continuouslychanging equilibrium with the upflowing steam in the tubes 76. Theupwardly flowing steam exits the upper end of the tubes as vent gasstream H to be carried away through pipe 78 connected to upper chamber80. The condensate is directly removed through removal pipe 82 to areservoir 83 from which pump 84 draws. A pressure reducing valve 85equalizes the pressure of the two condensate streams flowing into thereservoir 83. Vapor produced by the pressure reducing operation istransported from the reservoir to the shell side of the heat exchangervia pipe 81. Make up liquid is received as necessary from pipe 86. Thepump supplies condensate from the reservoir via pipe 88 to the shellside of the tube bundle, where the condensate is distributed on thetubes. One method of distribution is the spray nozzle 89. Clean steamcreated on the shell side of the heat exchanger is removed through pipe90 and sent to the process plant. Liquid condensate not evaporated intoclean steam is collected at the bottom of the shell side and removed viapipe 92 back to the reservoir.

In the operation of a counter current flow system as illustrated in FIG.4 wherein the noncondensable gases are stripped on the tube side of theheat exchanger it may be preferable to place the tube bundle betweenvertical and an acute angle to the horizontal, such as between zero andfifteen degrees with a low angle such as five degrees being desirable.The nearly horizontal direction of the tubes permits the coolingcondensate to drip through the tube bundle to reach all tubes in thetube bundle.

Crossflow may be applied to a system that condenses steam on the outsideof the heat exchanger tubes. FIG. 5 illustrates a heat exchanger 100 ofthe shell tube type wherein the geothermal steam stream A is receivedthrough pipe 102 near the bottom of the exchanger and travels upwardlythrough the vertical tube bundle to vent pipe 104 near the top of theexchanger. In this arrangement, the condensate formed on tubes 106drains by gravity against the flow of steam. Baffle plates 108 may beused to interrupt the steam path by causing the steam to travel fromside-to-side as it travels toward the top of the shell, and these platesare preferably spaced with increasing closeness toward the top of theshell, corresponding to the decrease in steam quantity because of thecondensation that has taken place at lower levels. This arrangementincreases the efficiency of the noncondensable gas stripping process.Condensate is gathered at the bottom of the shell volume and removedfrom the shell by pipe 110 to chamber 112 maintained at a lower pressurethan the shell side of the exchanger by valve 114 in pipe 110. Chamber112 is both a flash chamber and storage reservoir for condensate, whichis then drawn off through pipe 116 to pump 118 and pumped to upperchamber 120, which is connected to the tube side of the exchanger. Thecondensate drains through the tubes from chamber 120 to chamber 112,cooling the geothermal steam while itself being evaporated into cleansteam, which is drawn from the lower chamber by pipe 122 leading to theprocess plant or other end user. Blowdown tube 124 permits disposal ofexcess condensate from chamber 112. In this configuration anintermediate condensate flash tank could be utilized in pipe 110, asdescribed earlier with reference to FIG. 1, to flash and segregate asmall portion of vapor containing most of the noncondensable gasesdescribed in the shell side condensate.

If the tube bundle of FIG. 5 were horizontally disposed, the geothermalsteam would flow upwardly in the shell and perpendicular to the tubeaxes, and vent gas would again be removed from the top of the shellvolume. Condensate forming on the exterior of a horizontal tube woulddrain to the bottom side of the tube and drip to the lower tubes,finally reaching the liquid sump in the bottom of the shell. Coolingcondensate may be supplied to the horizontal tubes by a pump, and otherparts arranged to accomodate the variation in the orientation of thetube bundle with respect to the direction of gravitational force fromthe arrangement of FIG. 5.

The process and equipment previously described is applicable togeothermal steam feeding or leaving a process plant. A similarapplication can be made for a single geothermal well or a set of wellsthat are under test or for other reasons are blowing steam to theatmosphere. In this instance, a portable heat exchanger is connected tothe steam output from the single well or set of wells. The system willoperate as previously described for FIG. 1, except that the clean steam,instead of being directed to a process plant, will be discharged to theatmosphere. Since optimization of heat recovery is not a significantfactor in this case, a much smaller heat exchanger may be employed thanis required for a process plant. When a portable exchanger is being usedon a single well or set of wells, the vent gas stream H must be disposedof in a satisfactory way. There may be no suitable H₂ S recovery systemsuch as a Klaus plant in the near vicinity of the well or wells as therewould be in the vicinity of a process plant. In this instance, theconcentrated gases could be injected into another disposal well.

I claim:
 1. The method of producing a clean steam from a gaseous mixtureof steam and noncondensable gas including H₂ S by separating thenoncondensable gas from the steam in a continuous process,comprising:(a) condensing steam from the gaseous mixture of steam andnoncondensable gas on the first side of a two sided heat exchanger toform a condensate and a vent gas by(1) directing the gaseous mixturealong a flow path having condensing elements therein, and (2) directingthe condensate formed on said condensing elements in a substantiallycounter current flow to said flow path of the gaseous mixture so thatthe uncondensed gaseous mixture following the flow path stripsnoncondensable gases including substantially all of said H₂ S from thecondensate; (b) removing the vent gas from the first side of the heatexchanger; (c) removing the condensate from the first side of the heatexchanger; (d) evaporating a portion of the condensate so removed bysubjecting the condensate to a lower pressure than the pressure at whichthe gaseous mixture condenses; and (e) cooling the second side of theheat exchanger with the liquid condensate remaining after theevaporating step while evaporating said remaining condensate into aclean steam containing relatively less noncondensable gas than saidoriginal gaseous mixture.
 2. The methed of claim 1, wherein the heatexchanger comprises a tube bundle axially oriented non-horizontally andreceiving the gaseous mixture into the lower ends of the tubes in thebundle for gravity induced counter-current flow of the condensate out ofthe lower ends of the tubes, and wherein the method further comprisescollecting the condensate from the lower ends of the tubes anddistributing the collected condensate on the outside of the tubes. 3.The method of claim 1, wherein said tube bundle is axially oriented atan angle between zero and fifteen degrees to the horizontal and thecondensate is applied from the vertically top side of the tubes.
 4. Themethed of claim 1, wherein said heat exchanger comprises a substantiallyvertical tube bundle surrounded by a shell, and the flow path is fromthe bottom to the top of the heat exchanger on the shell side thereof.